Paging Data
Paging is a big issue for developers as it is required for many applications but entails many performance problems. Developers have used different workarounds to support paging.
For example, assume we need to display several pages of the HumanResource.Employee object with 5 rows per page. Below is the query you need to execute this in SQL Server 2012.
DECLARE @page_index SMALLINT ,
@page_Size SMALLINT = 5,
@offset INT
SET @page_index = 2
SET @offset = ( (@page_index - 1) * @page_Size)
SELECT BusinessEntityID,
NationalIDNumber,
JobTitle
FROM HumanResources.Employee
ORDER BY BusinessEntityID
OFFSET @offset ROWS
FETCH NEXT @page_Size ROWS ONLY
In this query you will note that there are two additional syntaxes added to the well-known SELECT query.
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OFFSET <offset> ROWS
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The number of rows to exclude from the top.
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FETCH NEXT <page_size> ROWS ONLY
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The number of rows to display.
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So in the results you will only receive the data you need.
You must remember that ORDER BY clause is a must, you can have multiple columns for the ORDER BY clause.
SELECT SOH.SalesOrderID
,SOD.SalesOrderDetailID
,P.Name As ProductNam
FROM
Sales.SalesOrderDetail SOD INNER JOIN Sales.SalesOrderHeader SOH
ON SOD.SalesOrderID = SOH.SalesOrderID
INNER JOIN Production.Product P ON P.ProductID = SOD.ProductID
ORDER BY SOH.SalesOrderID,SOD.SalesOrderDetailID DESC
OFFSET 100 ROWS
FETCH NEXT 25 ROWS ONLY
The query plan for this will appear as below:
As you can see, the Top operator will be introduced at an almost zero percent cost.
Lag & Lead
This is another T-SQL feature added to SQL Server 2012 addresses an issue in reporting.
Let me describe this feature with a real world example.
Below are the sales amounts for one month.
The above data was generated from the FactInternetSales table in the AdventureWorksDWDenali database. You can find the same script in the 5528EN _02_03.sql file in the code folder.
For reports you need this month’s sales and last month’s sales along with the difference between the two months.
If you were asked to write a query for this, you query would look like:
SELECT
t1.Year,
t1.Month,
t1.SalesAmount as SalesAmountThisMonth,
t2.SalesAmount as SalesAmountLastMonth,
t1.SalesAmount - t2.SalesAmount SalesDifferent
FROM MonthlySales as t1
LEFT OUTER JOIN MonthlySales as t2
ON (t1.Year = t2.Year) AND (t1.Month = t2.Month+1)
ORDER BY Year, Month
You need to use LEFT OUTER JOIN as INNER JOIN will not give any rows for the very first month in the data set.
Obviously, there will be a either an index scan or a table scan (depending whether you have an index or
not) twice for the table which is costly.
With SQL Server 2012, the new T-SQL function LAG is introduced just to address the above scenario:
SELECT
Year,
Month,
SalesAmount as SalesAmountThisMonth,
LAG(SalesAmount, 1, NULL) OVER(ORDER BY Year, Month) as SalesAmountLastMonth,
SalesAmount - LAG(SalesAmount, 1, 0) OVER(ORDER BY Year, Month) SalesDifferent
FROM MonthlySales
As shown in the above example, the LAG analytical function has three parameters. The first parameter is the expression which is the column in this scenario. The second parameter is the offset, since we are looking for last month this parameter is 1. If you want the last year’s comparative month’s data to be displayed then this parameter should be 12. The third parameter is the default, in case there is no match for the previous month, default value will be displayed.
So the output of the above query will be:
So how does this new syntax compare with the pre-2012 syntax? The simple way to compare them is running both queries in one batch and check the execution plan as shown in the below image. In the batch, the first query is the old style query and second query is the one using the LAGfunction.
Relative cost wise, the first query (old style LEFT OUTER JOIN) cost is 77% whereas the query which contains LAG cost is 23%. These queries were analyzed for Reads and duration using the SQL Profiler.
Below are the results for both queries executed after clearing the cache.
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Reads
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Duration (ms)
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LEFT OUTER JOIN
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125
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8
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LAG
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46
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3
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Obviously using LAG has a clear performance advantage. However, to achieve better performance you need to cover those columns (in the above scenario, it will be Year and Month).
You can also use the LAG function for grouping. For example, if you want product model sales differenciated by name you could use the below query.
SELECT
ModelName,
Year,
Month,
SalesAmount as SalesAmountThisMonth,
LAG(SalesAmount, 1, NULL) OVER(PARTITION BY ModelName ORDER BY Year, Month) as SalesAmountLastMonth,
SalesAmount - LAG(SalesAmount, 1, 0) OVER(PARTITION BY ModelName ORDER BY Year, Month) SalesDifferent
FROM ModelNameWiseMonthlySales
And the result is:
From the above results, you will see that the Sales Amount for the previous month is taken only for the model name.
LEAD is another new function which is similar to LAGbut instead shows the future records.
SELECT
Year,
Month,
SalesAmount as SalesAmountThisMonth,
LAG(SalesAmount, 1, NULL) OVER(ORDER BY Year, Month) as SalesAmountLastMonth,
LEAD(SalesAmount, 1, NULL) OVER(ORDER BY Year, Month) as SalesAmountNextMonth
FROM MonthlySales
As you can see from the below screenshot, the above query will give you the previous month’s and next month’s sales values in a single query.
FIRST_VALUE & LAST_VALUE
FIRST_VALUE will return the first value while LAST_VALUE will return the last in an ordered set of values in SQL Server 2012.
Take for example, the below query.
SELECT
d.LastName,
d.GroupName,
e.JobTitle,
e.HireDate,
FIRST_VALUE(e.HireDate) OVER (PARTITION BY d.GroupName ORDER BY e.JobTitle)
AS FirstValue,
LAST_VALUE(e.HireDate) OVER (PARTITION BY d.GroupName ORDER BY e.JobTitle)
AS LastValue
FROM HumanResources.Employee e
INNER JOIN HumanResources.vEmployeeDepartmentHistory d
ON e.BusinessEntityID = d.BusinessEntityID
This query will join the HumanResources.Employee table and the HumanResources.vEmployeeDepartmentHistory view together via BusinessEntityID. You can see that LastName, JobTitle, GroupName and Hiredate are selected from either the table or the view.
If you closely look at the LAST_VALUE and the FIRST_VALUE function, you will observe that the Partition column and the order by column (Which are HireDate and GroupName respectively) are the same.
FIRST_VALUE will give you the first value of the hiredate for each group when it orders by job title.
The results are given below:
CUME_DIST / PERCENT_RANK
CUME_DIST and PERCENT_RANK can be used to calculate the relative rank of a row within a group of rows in SQL Server 2012.
Use PERCENT_RANK to evaluate the relative standing of a value within a query result set or partition.
PERCENT_RANK = (RANK() – 1) /
(TotalRows – 1)
CUME_DIST calculates the relative position of a specified value in a group of values.
The below example displays the Rank, PERCENT_RANK and CUME_DIST
SELECT SalesOrderID,
RANK() OVER(ORDER BY SalesOrderID) RANK,
PERCENT_RANK() OVER(ORDER BY SalesOrderID) AS PERCENT_RANK,
CUME_DIST() OVER(ORDER BY SalesOrderID) AS CUME_DIST
FROM Sales.SalesOrderDetail
WHERE SalesOrderID IN (75121,75122,75123 )
The result for the above query is shown below:
IIF
I’m not sure why the IIF function which is a very simple function has not been included in previous versions of SQL Server. IIF is similar to the CASE function but it is optimized for evaluating two values. For example:
SELECT PersonType,
IIF(PersonType = 'EM','Employee','Other') PersonType
FROM Person.Person
The above query will return an Employee for the rows where PersonType equals EM and Other if not.
CHOOSE
This new simple feature is useful for extracting data elements from a data array:
DECLARE @MonthNumber int = 5
SELECT CHOOSE(@MonthNumber, 'JAN','FEB','MAR','APR','MAY','JUN','JUL','AUG','SEP','OCT','NOV','DEC') AS Month
The above will return the 5th element from the above array which is MAY.
TRY_CONVERT
The CONVERT function was used for earlier versions of SQL Server.
Let us say, we have a table with dates.
INSERT INTO MyData
(MyDate)
VALUES
('2011-01-01'),
('2012-12-31'),
('2012-02-29'),
('2011-02-29')
Note that 2011-02-29 is an invalid date. You could use CONVERT function to convert varchar to the date data type:
SELECT CONVERT(date,MyDate) Mydate
FROM MyData
However, will return an error since you have an error in the last row. In SQL Server 2012, you can use the new feature, TRY_CONVERT.
SELECT TRY_CONVERT(date,MyDate) Mydate
FROM MyData
TRY_CONVERT will return all the correct rows and invalid row will return null value. Main important thing is this won’t trigger an exception which will fail.
Apart from this you can use TRY_CONVERT function to retrieve only the error records.
SELECT MyDate
FROM MyData
WHERE TRY_CONVERT(date,Mydate) IS NULL
Instead of throwing an error TRY_CONVERT returns a NULL value and you can then more easily handle this in your code.
CONCAT
CONCAT is a significant addition for DQL Server developers.
If you have table with Title, FirstName, MiddleName, LastName columns and you may need to combine these columns and display them as a single column as shown below.
SELECT Title,FirstName,MiddleName, LastName ,
Title + ' '+ FirstName+ ' '+MiddleName+ ' '+LastName FullName
FROM Person.Person
Result are shown below:
However, except for three records and all the rows are null. That is because when a NULL value is added to a value the result is NULL.
The solution is to utilize the ISNULL function.
SELECT Title,FirstName,MiddleName, LastName ,
ISNULL(Title,'') + ' '+ FirstName+ ' '+ ISNULL(MiddleName, '' )+ ' '+LastName FullName
FROM Person.Person
However, this might be a tedious task if you have numerous columns to combine. In SQL Server 2012, you have a new function CONCAT which returns a string that is the result of concatenating two or more string values.
SELECT Title,FirstName,MiddleName, LastName ,
CONCAT(Title, ' ',FirstName, ' ',MiddleName, ' ',LastName) FullName
FROM Person.Person
From the below results you can see the null issue is eliminated in the FullName column.
Note that the CONCAT function has no performance impact compared with the old-style syntax.
PARSE & TRY_PARSE
PARSE is somewhat similar to CONVERT in that it can convert one value to another specified value, however it should only be used for converting from a string to a date/time or number type. Note that since this is a CLR function it may have some performance impact.
SELECT PARSE('11/28/2011' AS datetime)
GO
SELECT PARSE('Monday, 28 November 2011' AS datetime USING 'en-US')
GO
As with TRY_CONVERT there is a function called TRY_PARSE to facilitate errors – ie it returns a NULL instead of throwing an error.
Date Time Functions
There are quite few date time functions introduced as shown in the below code. All the below functions will take parameters and generate datetime values from (except for DATEPART which outputs a part of a date as an integer).
SELECT
DATEFROMPARTS(2012, 01, 31) AS DATE_FROM_PARTS,
DATETIME2FROMPARTS(2012, 01, 31, 15, 24, 5, 2, 7) AS DATETIME2_FROM_PARTS,
DATETIMEFROMPARTS(2012, 01, 31, 15, 30, 5, 997) AS DATETIME_FROM_PARTS,
DATETIMEOFFSETFROMPARTS(2012, 01, 31, 15, 30, 5, 1, -8, 0, 7) AS DATETIMEOFFSET_FROM_PARTS,
SMALLDATETIMEFROMPARTS(2012, 01, 31, 15, 30) AS SMALLDATETIME_FROM_PARTS,
TIMEFROMPARTS(15, 30, 5, 1, 7) AS TIME_FROM_PARTS
All above functions will add different date parts and outputs will be given as a datetime.
EOMonth
In many human resource applications, you are required to get last date of the month. In previous versions of SQL Server, you might need to use following syntax.
SELECT
CAST(DATEADD(d,-1,(DATEADD(mm,DATEDIFF(m,0,GETDATE())+1,0)))AS DATE)
In SQL Server 2012 you can simply use EOMONTH to perform this:
SELECT EOMONTH(GETDATE())
There will not be any performance gain or loss since this function and the old-style function will both have same performance impact.